This invention relates to a process for improving the octane number of a naphtha cut recovered from heavy petroleum products, e.g., a cut recovered from treatments applied to crude oils (treatments such as the cracking of heavy products), or of a naphtha cut recovered from a fractionation zone for oils obtained by liquefaction of coal with hydrogen.
A naphtha cut of this type, used as feed charge, distills at between about 25.degree. and 230.degree. C. (if necessary between 30.degree. and 220.degree. C.) and comprises usually at least 70% b.w. of (alkylated or not alkylated) naphthenic (cycloparaffinic) hydrocarbons comprising 3 to 30 carbon atoms per molecule; it generally also contains one or more compounds selected from the sulfur compounds, the nitrogen compounds and the oxygen compounds, this cut having a content by weight of at least 1000 ppm, as sulfur, of sulfur compounds, and/or of at least 1000 ppm, as nitrogen, of nitrogen compounds, and/or of at least 2000 ppm, as oxygen, of oxygen compounds.
The usual method to upgrade a naphtha of this type and to obtain, for example, motor-grade gasoline, is to subject it to catalytic reforming. However, since the catalytic reforming reactions necessitate catalysts of outstanding stability and life time, more and more sophisticated bifunctional catalysts have been used, for example bimetallic catalysts, which contain, in addition to an acid carrier, two metals usually belonging to the platinum family, which catalysts now often contain 3 or more metal elements. To avoid a premature decrease of the stability and life time of the reforming catalysts (which are sensitive to the sulfur, nitrogen and oxygen impurities of the charge subjected to reforming), it is necessary that the charge, before admission into the reforming zone, be subjected to a hydrotreatment performed under severe conditions, particularly under a high pressure of, for example, about 100 to 120 bars, to decrease the sulfur content of the charge to less than about 20 ppm, the nitrogen content to less than about 10 ppm and the oxygen content to less than about 15 ppm. As a matter of fact, outstanding results in the reforming zone are only obtained if the content of each of the oxygen, nitrogen and sulfur elements is not higher than 1 ppm. Further, at the outlet of the hydrotreatment zone, the charge is dried before being supplied to the reforming zone.
Finally, the reaction of catalytic reforming, irrespective of the preliminary hydrotreatment, must be effected under relatively severe conditions: operation performed at between 510.degree. and 600.degree. C., under a pressure of 1 to 15 bars, at an hourly velocity of 1 to 10 volumes of liquid charge per volume of catalyst, the hydrogen/hydrocarbon ratio being from 5 to 20.
The reforming catalyst has both a dehydrogenation function and an isomerization function. The first function is conferred by the metals of the catalyst and the second function results from the acidic properties of the catalyst.
These acidic properties are generally intrinsic to the selected carrier; they are however generally increased and/or modified by introduction of elements such as fluorine, chlorine, etc.
However, the acidic function of the catalyst also promotes certain undesirable reactions such as cracking or formation and deposit of carbon or coke on the catalyst, which results in a quick deactivation of the catalyst and consequently in a significant decrease of the yield of the resultant products and in the formation of hydrogen contaminated with hydrocarbons such as methane, ethane, etc.